The present invention is concerned with hydraulic fracturing of ore bodies and, more particularly, with the hydraulic fracturing of ore bodies mined by caving especially block caving.
Caving is a technique of mining wherein an ore body or rock mass is undercut under a sufficient area that the material "caves" from the bottom of the undercut area, referred to as the "block". Broken material is progressively drawn off and the caving of the mass continues upward through the ore body. The rate at which this caving action progresses is dependent upon the rate at which broken material is drawn off.
Caving, where the ore body is suitable, gives a lower mining cost per tonne than any other underground method. In contrast to other methods there is relatively little drilling, blasting and rock support done per tonne of ore, but nevertheless the preparation of the blocks for caving requires considerable time and large expense. For this reason the technique is best suited to wide veins, thick beds or massive deposits of homogeneous ore, overlain by ground which will cave readily. Ore bodies where the ore is soft or highly fractured and breaks fine are most suitable.
In ore bodies that are marginally cavable it is possible that, instead of continuously caving, a stable arch can form if the rock mass is strong enough. It is then difficult to promote further caving and the stable arch must be broken up. This has been observed, for example, in the Urad mine in the late 1960's. Production started in July 1967 and about 40,000 square feet of a portion of the ore body 750 feet long and 300 feet wide was undercut. By November 1967 it was realised that there was a problem with caving, and in December 1967 it was discovered that a stable arch had formed and that there was no caving above the arch. From January 1968 to October 1968, drilling and blasting were tried in several unsuccessful attempts to bring down the arch. Although the arch was ultimately brought down in this way, it is estimated that the total cost of the operation was around $2,000,000.
The present invention seeks to reduce the cost of caving and provide a means of avoiding and/or overcoming problems associated with caving stronger rock by utilising the technique of hydraulic fracturing. Hydraulic fracturing is a technique used in the petroleum industry and more recently the mining industry but has not been successfully applied to caving. In the petroleum industry, hydraulic fracturing is used to connect the well to a larger volume of the reservoir rock formation through a conductive fracture, resulting in an increased rate of hydrocarbon production from a well. Hydraulic fracturing has also been used to fracture coal seams prone to gas bursts, to release the gas from the seam and avoid "bumping". A typical disclosure of such a process occurs in Russian patent application number 1234658.
Hydraulic fracturing and water infusion have also been used in coal mining as a way of weakening the rock immediately above the coal over mined out parts of the seam to cause this rock to fail and form gob or goaf as described by a paper titled "A Study to Determine the Feasibility of High Pressure Water Infusion for Weakening the Roof" by J W Summers and E Wevell that was presented at the 2.sup.nd AAC Mining Symposium in 1985. Although fluid pressures of up to 9 megapascals were reached, the rate of fluid injection used was less than 5 liters/minute.
Moreover, hydraulic fracturing is a technique used in shaftless mining of minerals, wherein a rock formation is broken and then a leaching solution is injected into the deposit. The leaching solution is recovered and includes mineral values.
Russian patent application number 1164416 describes a process for preparing forward rock for driving which comprises injecting a mineral binder into drill holes in the rock, installing charges in the holes and detonating the charges, then pumping an aqueous surfactant solution into the same holes to hydraulically fracture the rock. This process speeds up heading operations by predisposing the forward rock to breakage. However, there is no disclosure of any caving technique in this patent and, in any event, hydraulic fracturing is only attempted after the rock has been first drilled and blasted.
Russian patent application number 1029677 discloses a process for rock breaking which consists of creating an additional free face, drilling a row of holes in the lock and breaking the rock out in slices onto the free face. However, before breaking the rock out, all holes in the block are hydraulically fractured. Once the rock has been hydraulically fractured it opens out and creates cracks to reduce pressure, and the equipment such as a wedge and piston and breaker jaws are used to break down the rock formation. The rock formation does not collapse under its own weight as in block caving.
Injection of water into the rock to reduce the effective normal stress in the rock was first tried independently by Northparkes Mines in late 1997, but this method had no effect on caving. The equipment used and techniques tried did not result in any hydraulic fractures forming.